The E2F family of transcription factors are encoded by eight distinct genes that based on structure-function studies and amino acid sequence analysis, fall into two main subclasses, activator E2Fs (E2F1-3) and repressor E2Fs composed of canonical repressors (E2F4-6) and atypical repressors (E2F7-8). Unlike other E2F family members, E2F7 and E2F8 bind DNA independent of dimerization with DP1/DP2 proteins and lack amino acid sequences typically used to interact with Rb-related proteins, and thus these atypical E2Fs may function outside the canonical CDK-Rb-E2F pathway. With the recent development of key genetic tools to conditionally disrupt or express E2f7 and E2f8 in mice, knocking mice that conditionally express endogenous wild type and mutant forms of E2F8 protein and knockin mice containing altered promoters of two key E2F-responsive target genes (Cdc6 and Cyclin A2) we expect to make significant strides towards a mechanistic understanding of how this important arm of the E2F family contribute to the control of transcription, cell cycle, and tumor suppression in vivo. Key preliminary data shows that E2F8 plays a critical role in endocycles and tumor suppression in the mouse liver and that its activity appears to be mediated by physical interactions with E2F7 and associated macro-molecular complexes to regulate gene expression outside the influence of the Cdk-Rb pathway. The overarching hypothesis of this proposal is that E2F8 functions as a transcriptional repressor to control cell cycles, genome ploidy levels and acts as a tumor suppressor in HCC. Three specific aims utilizing genetic, biochemical, and global profiling approaches will directly test this hypothesis. The long-term goal of these studies is to understand how the atypical (E2F8) arm contributes to the overall E2F transcriptional program in controlling cell cycles and tumor suppression.